1. Directionality, LEDs have viewing angles of 15 to 150 degrees depending on the lens usually while most used in horticulture are in the 60 to 120 degree range. HIDs radiate in a 360 degree range meaning that reflectors are required along with accompanying losses as heat. Some HID growers have overcome this limitation by doing vertical grows where no reflector is required;
2. Long life, 50,000 hours vs. 10,000 - 20,000 hours for HID with ever decreasing light;
3. Spectrum can be custom designed;
4. Lumen decay is slower than with HID. Translated, this means that LEDs will hold closer to their initial brightness throughout their life than HIDs. The reasons are simple. HIDs have metal filaments that get extreme heat stress while the gasses contained in the bulb may also age and leak. Compare that to LEDs, which are solid state semi-conductors with few parts and subject to a much narrower range of heat-stress.